1. Field of the Invention
The present invention relates to a fuel injection device of an internal combustion engine, specifically, a diesel engine. Specifically, the present invention relates to a fuel injection device that has a pressure intensifying mechanism.
2. Description of Related Art
A common rail system is known as a fuel injection device for a diesel engine. The common rail system has a common pressure accumulator to accumulate pressurized fuel pressure-fed from a fuel supply pump and injects the accumulated fuel into respective cylinders by opening and closing injection nozzles with oil pressure control valves. The common rail system has excellent properties such as an ability to control injection pressure and injection amount independently.
In recent years, further improvement of performance of the common rail system has been required from the viewpoint of exhaust emission purification or fuel consumption improvement. In order to respond to this requirement, a newly proposed system has a pressure intensifying mechanism that raises the fuel injection pressure and a mechanism that controls a nozzle opening-closing operation with oil pressure, which is an advantage of the common rail system, for example, as described in Japanese Patent No. 2885076.
This fuel injection device can perform the injection at higher pressure than before with the use of the pressure intensifying mechanism. In addition, this system can change the injection pressure in one injection cycle by controlling both of the pressure intensification and the injection. Further, this fuel injection system enables multiple injection modes such as a minute amount injection at low pressure or a main injection at extra-high pressure. Accordingly, fine control corresponding to an operating state can be performed to optimize combustion.
Since this kind of system essentially has to control the two operations, i.e., the pressure intensifying operation and the injection operation, respectively and independently, the system has to have at least two actuators. Accordingly, the structure of the system tends to be complicated, increasing a cost. It has been required to realize the similar function with an easier structure.
Another system described in JP-A-2003-106235 selectively opens and closes flow passages leading to a nozzle back pressure chamber and a pressure intensification control chamber with a control valve. A degree of control freedom is limited, but only one actuator is used and an entire structure is simplified.
FIG. 11 is a schematic diagram showing the fuel injection device described in JP-A-2003-106235. The fuel injection device 1 has a pressure accumulator 10, a pressure intensifier 20, an injection nozzle 30, a check valve 40 and a control valve 50 for controlling operations of the parts 10-40. The pressure intensifier 20 intensifies pressure of fuel in a high-pressure chamber 24 by driving a pressure intensifying piston provided by a large diameter piston 21 and a small diameter plunger 22 and supplies the fuel to the injection nozzle 30. Several restrictors are provided in fuel passages connecting the parts 10-50. For example, a control chamber 25 of the pressure intensifier 20 communicates with the pressure intensifier 10 through a restrictor 91 and communicates with the control valve 50 through a restrictor 92. Thus, pressure supply to the control chamber 25 and pressure release to a low-pressure system are controlled. A back pressure chamber 34 of the injection nozzle 30 communicates with the pressure accumulator 10 through restrictors 91, 92, 93 and communicates with the control valve 50 through the restrictor 93. Thus, pressure supply to the back pressure chamber 34 and pressure release to the low-pressure system are controlled.
In the above-described structure of the related art, the single control valve 50 controls pressures of the two chambers, i.e., the control chamber 25 of the pressure intensifier 20 and the back pressure chamber 34 of the injection nozzle 30. Therefore, the above structure is provided as an oil pressure circuit for connecting these elements. Even if the three restrictors 91, 92, 93 are used, it is difficult to optimize all of the pressures. When the control valve 50 is opened, a large amount of the fuel is continuously discharged from the pressure accumulator 10 due to the pressure intensifying control. Characteristics of a pressure-feeding stroke and a returning stroke of the pressure intensifier 20 can be set respectively but restrictor values are subject to restraint because the characteristics are not independent of injection characteristics. As a result, the optimization becomes further difficult.
As described above, the pressure in the control chamber 25 of the pressure intensifier 20 and the pressure in the back pressure chamber 34 have to be controlled. However, since the oil pressure circuit connects these chambers 25, 34 with each other, the pressure intensifying operation and the injection operation occur at the same time necessarily. Therefore, the injection cannot be performed at low pressure, i.e., without intensifying the pressure. As a result, multi-step injection at low pressure, which is preferable from the viewpoint of exhaust emission purification, cannot be performed, for example. Optimization of combustion through more sophisticated control has been required.